1. Field of the Invention
This invention relates to a multibeam scanning optical apparatus and also to a color image-forming apparatus. A multibeam scanning optical apparatus is an apparatus adapted to optically scan the surface of an object by means of a plurality of light beams that are emitted from one or more than one light sources, deflected by an optical deflector and transmitted by way of a scanning optical system having an fxcex8 characteristic. A color image-forming apparatus is an apparatus adapted to record image information obtained by scanning the surface of an image carrier by means of deflected light. Such apparatus find applications in the field of laser beam printers (LBPs) and digital copying machines having a feature of carrying out a color electrophotography process.
2. Related Background Art
In conventional scanning optical apparatus to be used for image-forming apparatus such as laser beam printers and digital copying machines, the light beam emitted from a light source and optically modulated according to the image signal applied to it is periodically deflected by a light deflector typically comprising a rotary polygon mirror and then focussed on the surface of a photosensitive recording medium (photosensitive drum) to produce a spot of light there by means of a focussing optical system having an fxcex8 characteristic, which optical system is then used to scan the surface of the-recording medium and record the image on the recording medium.
FIG. 1 of the accompanying drawings schematically illustrates a principal portion of a known scanning optical apparatus.
Referring to FIG. 1, a divergent light beam emitted from light source 81 is substantially collimated by a collimator lens 82 and restricted for quantity by a diaphragm 83 before it enters a cylindrical lens 84 that is made to have a predetermined refractive power only in the sub-scanning direction. The substantially collimated light beam entering the cylindrical lens 84 is then made to exit the lens as a beam substantially collimated in the main-scanning plane, while it is converged in the sub-scanning plane to produce a linear image on the deflection plane (reflection plane) 85a1 of a light deflector 85 comprising a rotary polygon mirror.
Then, the light beam deflected and reflected by the deflection plane 85a1 of the light deflector 85 is led to the surface 88 of a photosensitive drum to be scanned by way of a scanning optical element having an fxcex8 characteristic (fxcex8 lens) 86 so that the surface 88 of the photosensitive drum is optically scanned in the direction indicated by arrow 88a (main-scanning direction) to record the scanned image as the light deflector 85 is driven to rotate in the sense of arrow 85a. 
In order to accurately control the starting point of the operation of drawing the image for the scanning optical apparatus, the light beam deflected by the light deflector 85 is partly taken out and entered to beam detector (BD) sensor 92 by way of the scanning optical element 86, a beam detector (BD) mirror 95 and a slit 91 immediately prior to the start of writing the image signal. Then, the output signal of the beam detector (BD) sensor 92 is used to regulate the timing and the spot at which the operation of drawing the image on the surface of the photosensitive drum is started.
In recent years, as a result of technological development in the field of image forming apparatus involving the use of an electrophotography process particularly in terms of high speed and high resolution, there is an ever-increasing demand for multibeam scanning optical apparatus comprising a multibeam laser device having a plurality of light emitting sections and scanning optical apparatus of the type employing a plurality of scanning optical apparatus as so many units in order to realize high speed color image formation as shown in FIG. 2 that illustrates a tandem type color image forming apparatus where a plurality of scanning optical apparatus are operated simultaneously for different colors in order to record image information on respective photosensitive drums as well as hybrid type color image forming apparatus realized by combining apparatus of the above identified types. In FIG. 2, reference numerals 111, 112, 113 and 114 denote respective scanning optical apparatus and reference numerals 121, 122, 123 and 124 denote respective photosensitive drums operating as so many image carriers, whereas reference numerals 131, 132, 133 and 134 denote respective developing units and reference numeral 141 denotes a conveyor belt.
Since the manufacturing cost of such scanning optical apparatus is vital, the scanning optical element (fxcex8 lens) is typically prepared by plastic molding without the process of compensating, if any, the chromatic aberration of magnification.
However, in the case of a multibeam scanning optical apparatus adapted to form a final image by means of light beams of a multibeam laser having a plurality of light emitting sections, the plurality of light beams can show discrepancies in terms of magnification to consequently degrade the quality of the produced image due to various factors including those listed below:
(1) variances of the initial wavelengths of a plurality of light beams emitted from the multibeam laser;
(2) variances of the wavelength of the plurality of light beams caused by mode hopping of the multibeam laser that is attributable to environmental changes; and
(3) fluctuations in the refractive index of the plastic lens also attributable to environmental changes.
In FIG. 1, there are also shown the image region of the known scanning optical apparatus and the displacements of the focussing positions of the light beams as detected at the start of drawing the image when the wavelength of the light source of laser B (as observed by the laser beam emitted from the light source) is modified relative to the wavelength of the light source of the laser A. Note that, while only the focussing points of laser A and laser B are indicated by A and B respectively in FIG. 1, there are actually more focussing points that are not illustrated in FIG. 1.
In an actual image, the displacements of focussing positions on the surface being scanned due to variations of magnification (wavelength) do not give rise to any jittering along the left edge of the image but they do along the right edge of the image as shown in FIG. 3 to consequently degrade the recorded image. This is because the timing and the spot at which the operation of drawing the image on the surface of the photosensitive drum is started are regulated (synchronized) at the side of starting the scanning operation as pointed out above. More specifically, referring to FIG. 1, the detection light beam to be detected by the BD sensor 92 of the known scanning optical apparatus strikes aslant the fxcex8 lens 86 shared with the light beams for scanning the surface to be scanned and, if the wavelength of the laser beam emitted from the laser A differs from that of the light beam emitted from the laser B, they produce a relative deviation of the focussing positions equal to that of the starting points of scanning the surface to be scanned. Thus, the starting points of scanning of the light beams can be made to agree with each other by detecting a part of each of the light beams and so controlling the scanning timing of the beam as to correct the relative deviation of the focussing positions due to the difference of the wavelengths on the basis of the signals obtained by detecting the laser beams. However, it will be appreciated that, as a result, a remarkable jittering phenomenon can appear at the terminating points of scanning.
A similar problem arises in the scanning optical apparatus of tandem type color image forming apparatus. More specifically, when magnification discrepancies arises among a number of scanning optical apparatus, a relative deviation of registration occurs over the range from the center toward the right edge of the image as shown in FIG. 4 among different colors to consequently degrade the produced image. While FIG. 4 shows a relative deviation of registration between B (black) and C (cyan), a similar deviation can occur among any different colors.
In view of the above identified circumstances, it is therefore the first object of the present invention to provide a multibeam scanning optical apparatus that can effectively reduce jittering that can arise due to variations (deviations) of magnification among a plurality of light beams attributable to the difference of initial wavelength (wavelength deviation) among the plurality of light beams emitted from so many light emitting sections and environmental changes even when a molded plastic lens is used without being subjected to a process of correcting the chromatic aberration of magnification.
The second object of the present invention is to provide a color image forming apparatus having a simple configuration and free from the above pointed out problem of tandem type color image forming apparatus that a relative deviation of registration arises among different colors (color deviation) due to the difference of initial wavelength among the plurality of light beams emitted from so many scanning optical apparatus light emitting sections and environmental changes.
According to the invention, the above first object is achieved by providing a multibeam scanning optical apparatus comprising:
a light source having a plurality of light beam emitting sections;
a light deflector for deflecting a plurality of light beams emitted respectively from said plurality of light beam emitting sections of said light source;
a scanning optical system for focussing said plurality of light beams deflected by said light deflector on a surface to be scanned; and
a photodetector for controlling the timing of the start of scanning of said plurality of light beams by detecting a part of at least one of said plurality of light beams deflected by said light deflector as detection light beam;
said timing of the start of scanning being so controlled as to make the centers of the scanning areas of said light beams agree with each other on the surface to be scanned when said plurality of light beams have respective wavelengths that are different from each other.
According to the invention, there is also provided a multibeam scanning optical apparatus comprising:
a light source having a plurality of light beam emitting sections;
a light deflector for deflecting a plurality of light beams emitted respectively from said plurality of light beam emitting sections of said light source;
a scanning optical system for focussing said plurality of light beams deflected by said light deflector on a surface to be scanned;
a photodetector for controlling the timing of the start of scanning of said plurality of light beams by detecting a part of at least one of said plurality of light beams deflected by said light deflector as detection light beam; and
a detection optical element for converging said detection light beam and leading it to said photodetector;
said detection optical element having its optical plane arranged orthogonally relative to said detection light beam.
According to the invention, the above second object is achieved by providing a color image forming apparatus comprising:
a plurality of scanning optical apparatus, each having light source, a light deflector for deflecting a light beam emitted from said source, a scanning optical system for focussing the light beam deflected by said light deflector on a surface to be scanned and a photodetector for controlling the timing of the start of scanning of said light beam by detecting a part of said light beam deflected by said light deflector as detection light beam, said photodetector and the center of the scanning width in the main-scanning direction on the surface to be scanned being held optically equivalent; and
a plurality of image carriers arranged respectively on the surfaces to be scanned of said scanning optical apparatus for forming images with respective different colors.
According to the invention, there is also provided a color image forming apparatus comprising:
a plurality of scanning optical apparatus, each having light source, a light deflector for deflecting a light beam emitted from said source, a scanning optical system for focussing the light beam deflected by said light deflector on a surface to be scanned, a photodetector for controlling the timing of the start of scanning of said light beam by detecting a part of said light beam deflected by said light deflector as detection light beam and a detection optical element for converging said detection light beam and leading it to said photodetector, said detecting optical element having its optical plane arranged orthogonally relative to said detection light beam; and
a plurality of image carriers arranged respectively on the surfaces to be scanned of said scanning optical apparatus for forming images with respective different colors.
When the plurality of light beams show respective wavelengths that are different from each other in a multibeam optical apparatus according to the invention, the timing of the start of scanning is so controlled as to make the centers of the scanning areas of said light beams agree with each other on the surface to be scanned simply by controlling the timing of scanning in the main-scanning direction at or near the center of the scanning width in the main-scanning direction on the surface to be scanned by means of the photodetector (BD) sensor. This means that the photodetector and the center of the scanning width in the main-scanning direction on the surface to be scanned are held optically equivalent. Similarly, in each of the scanning optical apparatus of a color image forming apparatus according to the invention, the timing of the start of scanning is so controlled as to make the centers of the scanning areas of said light beams agree with each other on the surface to be scanned simply by controlling the timing of scanning in the main-scanning direction at or near the center of the scanning width in the main-scanning direction on the surface to be scanned by means of the photodetector (BD) sensor.
The photodetector and the center of the scanning width in the main-scanning direction on the surface to be scanned can be held optically equivalent by so arranging the detecting optical element for focussing the detection light beam and leading it to the photodetector (BD) sensor as to have its optical plane arranged orthogonally relative to said detection light beam. This is because the main light beam of the flux of light striking the center of the scanning width in the main-scanning direction generally agrees with the optical axis of the scanning optical system (fxcex8 lens) and hence this light beam and the light beam entering the photodetector (BD) sensor are made to optically equivalent.
For the purpose of the invention, the expression that xe2x80x9cthe detecting optical element having its optical plane arranged orthogonally relative to the detection light beamxe2x80x9d means that the main light beam of the flux of light as detected by the photodetector (BD) sensor substantially agrees with the optical axis of the detecting optical element. The expression xe2x80x9cas detected by the photodetectorxe2x80x9d is used because the light beam entering the photodetector is used for scanning and therefore the main light beam of the flux of light does not always agree with the optical axis of the detecting optical element. In other words, there exists a state where the main light beam of the flux of light being used for scanning can agree with optical axis of the detecting optical element.